In general, manufacturing steps for semiconductor devices involve forming a fine pattern by a photolithographic method. In the formation of this fine pattern, many substrates called transfer masks are typically used. Such transfer masks are generally transparent glass substrates on which a fine pattern formed of a thin metal film, for example, is provided. A photolithographic method is also used in the production of these transfer masks.
In the photolithographic production of transfer masks, mask blanks are used. The mask blanks have a thin film (for example, a light shielding film) for forming a transfer pattern (a mask pattern) on a transparent substrate such as a glass substrate. A transfer mask is produced from a mask blank through the steps including an exposure step in which a desired pattern is drawn in a resist film formed on the mask blank, a development step in which the resist film is developed in accordance with the desired pattern to form a resist pattern, an etching step in which a thin film of the mask blank is etched through the resist pattern, and a step in which the remaining resist pattern is removed by stripping. In the development step, after a desired pattern is drawn (by exposure) in a resist film formed on the mask blank, a developer is supplied and portions of the resist film which are soluble by the developer are dissolved away, thus forming a resist pattern. In the etching step, dry etching or wet etching is performed using this resist pattern as a mask to dissolve exposed portions of the thin film where the resist pattern is not formed. As a result, a desired mask pattern can be formed on the transparent substrate. A transfer mask is produced in this manner.
Fabricating finer patterns in semiconductor devices requires the miniaturization of mask patterns formed in transfer masks as well as that the wavelength of an exposure light source used in photolithography should be shorter. In recent years, exposure light sources in semiconductor device production have shifted towards shorter wavelengths, namely, from KrF excimer lasers (wavelength 248 nm) to ArF excimer lasers (wavelength 193 nm).
Heretofore, as transfer masks, binary masks which have a light shielding film pattern comprising of a chromium-containing material on a transparent substrate are known.
Binary masks designed for ArF excimer lasers have been recently developed which use a material containing a molybdenum silicide compound (a MoSi-containing material) as a light shielding film (Patent Literature 1: JP-A-2006-78807). Also binary masks for ArF excimer lasers which use a material containing a tantalum compound (a tantalum-containing material) as a light shielding film, are developed (Patent Literature 2: JP-A-2009-230112). Patent Literature 3 (JP-A-2010-192503) discloses that if a photomask which includes a light shielding film containing tantalum, niobium, vanadium or at least two metals of tantalum, niobium and vanadium is washed by acid washing or hydrogen plasma, the light shielding film undergoes hydrogen embrittlement and the film light shielding is sometimes deformed. According to the disclosure, this problem is addressed by forming a hydrogen block film which air-tightly covers the top and side faces of the light shielding film after patterning of the light shielding film.
On the other hand, Patent Literature 4 (JP-A-2004-26586) describes a process for producing mask substrates for vacuum UV lights from synthetic quartz glass. This literature indicates that it is necessary for the OH group content in synthetic quartz glass to be reduced in order to improve the transmittance of the synthetic quartz glass to vacuum UV lights. It is disclosed that one solution is to reduce the Si—H content and the H2 content in synthetic quartz glass to or below specific levels.